This section provides background information related to the present disclosure which is not necessarily prior art. This section provides a general summary of the disclosure, and is not a comprehensive disclosure of its full scope or all of its features.
In plasma spray processing, the material to be deposited (also known as feedstock)—typically as a powder, a liquid, a liquid suspension, or the like—is introduced into a plasma jet emanating from a plasma torch or gun. In the jet, where the temperature is on the order of 10,000 K, the material is melted and propelled towards a substrate. There, the molten/semi-molten droplets flatten, rapidly solidify and form a deposit and, if sufficient in number, a final layer. Commonly, the deposits remain adherent to the substrate as coatings, although free-standing parts can also be produced by removing the substrate. Direct current (DC) plasma processing and coating is often used in many industrial technology applications.
With particular reference to FIG. 1, a schematic of a conventional apparatus for conducting direct current plasma processing (FIG. 1(a)), as well as a photograph of the apparatus in operation (FIG. 1(b)), are provided. A conventional direct current plasma apparatus 100 generally comprises a housing 110 having a cathode 112 (which is negatively charged) and an anode 114 (which is positively charged). A plasma gas is introduced along an annular pathway 116 to a position downstream of cathode 112 and generally adjacent anode 114. An electrical arc is established and it extends from the cathode 112 to the anode 114 and generates the plasma gas to form a hot gas jet 118. Generally, this electrical arc rotates on the annular surface of the anode 114 to distribute the heat load. A precursor 120, such as in the form of a powder or a liquid, is fed from a position downstream of anode 114 and external to the plasma jet 118 into the jet boundary. This process is generally referred to as radial injection. The powders (solid) and/or droplets (liquid) within the precursor 120 are typically entrained into the plasma jet 118 and travel with it, eventually melting, impacting, and being deposited on a desired target. The powders are typically presynthesized by another process into a predetermined chemistry and solidified form and are typically sized on the order of microns.
Generally, the liquid droplets are typically of two types—namely, a first type where the liquid droplets contain very fine powders (or particles), which are presynthesized by another process into solid form being of submicron or nanometer size, suspended in a liquid carrier; and a second type where liquid droplets contain a chemical dissolved in a solvent, wherein the chemical eventually forms the final desired coating material.
In the first type, during deposition, the liquid droplets are entrained in the plasma jet 118, causing the liquid carrier to evaporate and the fine particles to melt. The entrained melted particles then impact on a target, thereby forming the coating. This approach is also known as “suspension approach”.
In the second type, as droplets travel in the plasma jet 118 a chemical reaction takes place along with the evaporation of the liquid solvent to form the desired solid particles which again melt and upon impact on the target form the coating. This approach is known as “solution approach”.
Generally speaking, the solid powder injection approach is used to form microcrystalline coatings, and both of the liquid approaches are used to form nanostructured coatings.
However, direct current plasma processing suffers from a number of disadvantages. For example, because of the radial injection method used in DC plasma processing, the precursor materials are typically exposed to different temperature history or profiles as they travel with the plasma jet. The core of the plasma jet is hotter than the outer boundaries or periphery of the plasma jet, such that the particles that get dragged into the center of the jet experience the maximum temperature. Similarly, the particles that travel along the periphery experience the lowest temperature. As seen in FIG. 2, a simulation of this phenomenon is illustrated. Specifically, the darker particles 130 are cooler, as illustrated by the gray scale, and travel generally along the top portion of the exemplary spray pattern in the figure. The lighter particles 132 are hotter, again as illustrated by the gray scale, and travel generally along the bottom portion of the exemplary spray pattern in the figure. This temperature non-uniformity of powder or droplets affects the coating quality negatively. This variation is especially disadvantageous in liquid-based techniques, which are typically used for nanomaterial synthesis.
Additionally, due to the radial injection orientation (see FIGS. 1(a)-1(b)), the entrained particles typically achieve a lower velocity due to the need to change direction within the jet from a radial direction (during introduction in the Y-axis) to an axial direction (during entrainment in the X-axis) and the associated inertias. This negatively affects the coating density and the deposition efficiency (i.e. amount of material injected compared to the amount that adheres to the target). Particularly, this is important for nanoparticle deposition as they need to achieve a critical velocity to impact upon the target forming the coating, lack of which would cause them to follow the gas jet and escape the target.
Further, the interaction time of the particle (related to the amount of heat that can be absorbed by the particle) with the jet 118 is shorter due to external injection and, thus, very high melting point materials that must achieve a higher temperature before becoming molten can not be melted by external injection due to the reduced residence time in the jet 118. Similarly, in the case of liquid precursors, lack of appropriate heating leads to unconverted/unmelted material resulting in undesirable coating structures as illustrated in FIG. 22.
Furthermore, the coatings typically achieved with conventional direct current plasma processing suffer from additional disadvantages in that as individual molten or semi-molten particles impact a target, they often retain their boundaries in the solidified structure, as illustrated in FIG. 3. That is, as each particle impacts and is deposited upon a target, it forms a singular mass. As a plurality of particles are sequentially deposited on the target, each individual mass stacks upon the others, thereby forming a collective mass having columnar grains and lamellar pores disposed along grain boundaries. These boundary characteristics and regions often lead to problems in the resultant coating and a suboptimal layer. These compromised coatings are particularly undesired in biomedical, optical and electrical applications (i.e. solar and fuel cell electrolytes).
Therefore, a need exists in the art for reliable ways to inject precursor material (either solid powder or liquid droplet or gaseous) axially within a jet 118 (i.e, in the same direction of the jet) to achieve improved coating results.
Accordingly, the present teachings provide a system for axial injection of a precursor in a modified direct current plasma apparatus. According to the principles of the present teachings, precursor can be injected through the cathode and/or through an axial injector sitting in front of the anode rather than radially injected as described in the prior art. The principles of these teachings have permitted formulation and the associated achievement of certain characteristics that have application in a wide variety of industries and products, such as battery manufacturing, solar cells, fuel cells, and many other areas.
Still further, according to the principles of the present teachings, in some embodiments, the modified direct current plasma apparatus can comprise a laser beam to provide an in-situ hybrid apparatus capable of producing a plurality of coating types. These in-situ modified coatings have particular utility in a wide variety of applications, such as optical, electrical, solar, biomedical, and fuel cells. Additionally, according to the principles of the present teachings, the in-situ hybrid apparatus can fabricate free standing objects comprising different materials such as optical lenses made using complex optical compounds and their combinations.
Further areas of applicability will become apparent from the description provided herein. The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.
Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings.